This invention relates to a method for determining the filter current limit of an electrostatic filter which is fed through a control element from an alternating current source and in which the control voltage of the control element can be varied up to a given filter current limit as a function of filter operating data.
A control method of this general type as described more specifically, for example, in the journal Siemens-Zeitschrift 1971, pp. 567-572.
Since the efficiency of an electrostatic filter increases approximately with the square of the applied voltage, it is desirable to set the filter voltage as high as possible. However, the breakdown strength of the gas sets an upper limit for this voltage. Since, apart from the disruptive discharge itself, there is no criterion for the maximum possible voltage, disruptive discharges must be brought about at certain intervals of time to sample this limit. Since the disruptive discharge limit can change very rapidly, one must sample relatively frequently.
With such a control dependent on disruptive discharge, provision must be made to insure that the current load capacity of the installation is not exceeded. In the known arrangement, an adjustable current limitation prevents further rise of the control voltage when the set value is reached. If the current increases because of a reduction of the dust resistance, the current limitation will bring about a slow falling of the control voltage until the current has dropped to its set value.
Besides this limitation of the nominal current, another type of filter current limitation is also of interest. Under certain operating conditions of an electrostatic filter, e.g. in sintering plants or in by-pass operation in cement mills, either a filter voltage maximum or a filter voltage saturation may occur.
Here the objective of the function "current limitation" is to establish the maximum or respectively the saturation, in order to prevent unnecessarily high filter currents.